Hydrogenation reactions typically involve the reaction of hydrogen gas with an organic compound to produce a hydrogenated organic compound. Hydrogenation reactions are important in a variety of chemical processes including the production of fuels and the conversion of sugars to the corresponding polyalcohols. Despite generally favorable thermodynamics, there is a large kinetic barrier that slows the reaction of hydrogen gas with many unsaturated and heteronuclear organic compounds. To speed the reactions, it is necessary to add a catalyst, such as a metal. The metals that catalyze hydrogenations are usually expensive, and therefore it is common practice to increase the reactive surface area by distributing the metals over the surface of a support, thus forming a supported metal catalyst.
In addition to a supported metal catalyst, many commercial hydrogenations also require the use of organic solvents that dissolve the organic compounds and promote contact with the solid catalyst. Organic solvents, however, are often toxic and can present problems in storage, transportation, and disposal. Thus it would be desirable to avoid the use of organic solvents in hydrogenation reactions.
Instead of organic solvents, it would be desirable to use water to dissolve the organic compounds and promote contact with a solid catalyst. Furthermore, some organic compounds, such as sugars, are more soluble in water than in organic solvents. Thus, in many cases it would be advantageous to conduct hydrogenations in the presence of water, i.e., in the aqueous phase. On the other hand, many existing catalysts are not appropriate for aqueous phase reactions because they do not provide optimal conversion efficiency and yield. Moreover, in applications outside the laboratory, catalysts should remain active after weeks or months of processing; however, in aqueous phase processing conditions, the catalytic activity of existing catalysts becomes degraded or destroyed.
Some catalysts designed for aqueous phase hydrogenation reactions have been disclosed in prior patents. For example, Boyers in U.S. Pat. No. 2,868,847 discloses examples in which hydrogenations were carried out over catalysts of ruthenium on carbon or ruthenium on alumina. Arena, in U.S. Pat. Nos. 4,380,679, 3,380,680, 4,413,152, and 4,503,274, discloses aqueous phase hydrogenations of various carbohydrates over a catalyst supported on carbonaceous pyropolymer, alpha-alumina, titanated alumina, and gamma-alumina, respectively. In U.S. Pat. No. 4,487,980, Arena discloses aqueous phase hydrogenations and an aqueous phase hydrogenation catalyst comprising ruthenium and a titania-containing support. Arena analyzed leaching from a titania-bentonite material and studied the aqueous phase hydrogenation of glucose to sorbitol. Arena did not disclose the stability of the catalyst and did not disclose a rutile-containing catalyst.
Ruthenium on titania catalysts have long been known for catalyzing non-aqueous phase reactions. For example, Arcuri et al., in U.S. Pat. No. 4,567,205, disclose a ruthenium on titania catalyst for use in Fischer-Tropsch catalysis in which carbon monoxide and hydrogen gas are reacted to produce hydrocarbons. Arcuri et al. used rhenium in the catalyst to improve activity maintenance in Fischer-Tropsch conditions. Arcuri et al. state that the rhenium:ruthenium weight ratio ranges from about 10:1 to about 1:10. Arcuri et al. also state that the titania support is preferred to have a rutile:anatase ratio of at least 2:3 and found that under Fischer-Tropsch conditions, a rutile:anatase ratio of 2:1 demonstrated superior activity maintenance as compared to ratios of 1.2:1 and 30:1.
Ongoing research at Battelle Pacific Northwest Division has produced numerous discoveries in the area of low temperature catalytic gasification of wet industrial wastes. Elliott et al. have tested various types of supports in high pressure, hot water and found that supports such as .gamma.- and .delta.-alumina and alumina-borate were unstable, but various other commercial supports such as titania and zirconia were reported to be relatively stable--see Low Temperature Catalytic Gasification of Wet Industrial Wastes, FY 1991-1992 and FY 1993-1994, pages B16-17 and 23, 25, respectively. Elliott et al., in U.S. Pat. No. 5,814,112, disclose a nickel/ruthenium catalyst on a porous support for aqueous phase reactions. It is suggested that the porous support could be alumina, titania in the rutile form, zirconia in the monoclinic form, granulated carbon, bohmite, or a commercial G1-80 catalyst.
Despite considerable efforts, there remains a need for aqueous phase hydrogenation catalysts that maintain catalytic activity for extended periods of time under hydrogenation conditions. There also exist needs for aqueous phase hydrogenation catalysts that exhibit excellent conversion efficiencies, high selectivities, operability at low temperatures, and/or high processing rates.